1. Field of the Invention
The present invention relates to a wave plate and an optical device using the same, and more particularly to a wave plate having linear grating groove patterns and an optical device using the same.
2. Description of the Background Art
In general, wave plate having linear grating groove pattern is known. A method employing photolithography and etching is known as a method of manufacturing the above conventional wave plate having linear grating groove patterns. This is disclosed in D. Hambach, G. Schneider and E. M. Gullikson “OPTICS LETTERS” Vol. 26, No. 15, Aug. 1, 2001, and pp. 1200–1202.
FIG. 93 is a perspective view showing the concept of a conventional wave plate having linear grating groove patterns. In the conventional wave plate 200, linear grating groove patterns are formed on a glass substrate 201, as shown in FIG. 93. The grating groove patterns are formed by air layers 202 and substrate material layers 203, having a width a, consisting of the same material as the glass substrate 201, and have a period P not more than the wavelength of light. It is assumed that the refractive indices of the air layers 202 and the substrate material layers 203 (the glass substrate 201) are 1 and n respectively. When light is incident upon the grating groove patterns of the wave plate 200, the wave plate 200 exhibits an effective refractive index corresponding to the mixture of the refractive indices 1 and n of the air layers 202 and the substrate material layers 203.
FIG. 94 is a correlation diagram showing the relation between the effective refractive index and the duty ratio of the conventional wave plate shown in FIG. 93. Referring to FIG. 94, the vertical axis shows the effective refractive index, and the horizontal axis shows the duty ratio (a/P), i.e., the ratio of the width a of the substrate material layers 203 to the period P of the grating groove patterns shown in FIG. 93. Further, symbol TE in FIG. 94 denotes light having a direction of polarization parallel to the extensional direction of the grating groove patterns, as shown in FIG. 93. Symbol TM denotes light having a direction of polarization perpendicular to the extensional direction of the grating groove patterns, as shown in FIG. 93.
Referring to FIG. 94, the effective refractive index varies with the duty ratio of the grating groove patterns. In this case, the effective refractive index of the light TE having the direction of polarization parallel to the extensional direction of the grating groove patterns differs from that of the light TM having the direction of polarization perpendicular to the extensional direction of the grating groove patterns. That is, when the duty ratio is D1, the effective refractive indices of TM and TE correspond to N1 and N2, respectively, as shown in FIG. 94. The characteristic of the effective refractive index varying with the direction of polarization of light is referred to as a birefringence property.
As a manufacturing process for the rectilinear grating groove patterns of the conventional wave plate 200 shown in FIG. 93, a method of forming rectilinear grating groove patterns by etching the surface of a glass substrate by photolithography and etching is conceivable, for example.
In the case of forming the rectilinear grating groove patterns of the conventional wave plate 200 shown in FIG. 93 by photolithography and etching, however, it is difficult to form grating groove patterns having a large depth with a uniform groove width along the depth direction. More specifically, grating groove patterns deeply formed by photolithography and etching have trapezoidal sections non-uniform in the depth direction as shown in FIG. 95, and hence duty ratios in upper and lower portions of the grating groove patterns disadvantageously differ from each other.
On the other hand, a process of manufacturing a triangular lattice pattern in the form of a regular triangle employing anodic oxidation is conventionally known. This manufacturing process is disclosed in H. Masuda et al. “Appl. Phys. Lett.” Vol. 71 (19), 10 Nov. 1997, and pp.2770–2772, for example. The process of manufacturing a triangular lattice pattern disclosed in this literature, capable of forming a triangular lattice pattern having deep and uniform micropores, is proposed as a process of preparing a two-dimensional photonic crystal. More specifically, a valve metal such as aluminum, titanium or tantalum or a semiconductor such as Si or GaAs has such a characteristic that an oxide film having micropores arranged perpendicular to the film surface is formed when an anode is electrified in an acidic electrolyte. In particular, an oxide film of aluminum has such a material characteristic that micropores are easily arranged in the form of a triangular lattice. A triangular lattice pattern having deep and uniform micropores can be formed through this characteristic.
FIGS. 96 to 99 are sectional views for illustrating a conventional process of manufacturing a triangular lattice pattern by anodic oxidation. FIG. 100 is a plan view showing a two-dimensional photonic crystal formed by conventional anodic oxidation. The conventional process of manufacturing a triangular lattice pattern by anodic oxidation is now described with reference to FIGS. 96 to 100.
In the conventional process of manufacturing a triangular lattice pattern by anodic oxidation, projecting portions 211a arranged in the form of a triangular lattice are formed on the surface of a press member 221 consisting of a hard material such as SiC, as shown in FIG. 96. Texturing is performed by pressing the press member 211 against the surface of an aluminum material 211. Thus, concave portions 211a arranged in the form of a triangular lattice are formed on the surface of the aluminum material 211, as shown in FIG. 97. Then, the aluminum material 211 formed with the concave portions 211a is oxidized in an electrolyte 222, as shown in FIG. 98. In this case, a cathode 223 is prepared from platinum or the like, and the electrolyte 222 is prepared from an aqueous solution of sulfuric acid, oxalic acid, phosphoric acid and so on. Thus, an aluminum oxide (alumina) film 212 having deep and uniform micropores 212a, starting from the concave portions 211a (see FIG. 97), arranged in the form of a triangular lattice is formed in a self-organized manner, as shown in FIGS. 99 and 100. The micropores 113a can be formed to have a depth of at least several hundreds μm with respect to submicron diameters.
However, the aforementioned conventional method of manufacturing a triangular lattice pattern by anodic oxidation has been known as a method of forming two-dimensional photonic crystal micropores. In general, therefore, there has been no attempt of forming grating groove patterns of the wave plate 200 shown in FIG. 93 by anodic oxidation.
As hereinabove described, it has been difficult to form a grating groove pattern having a large depth with a uniform groove width along the depth direction in general, and hence there is a problem that improvement of characteristics of wave plate with grating groove patterns is difficult.
In addition, a wave plate consisting of a birefringent material is also conventionally known. As birefringent materials composing this wave plate, quartz crystal with a birefringent crystal, a birefringent resin, and so on are known. When light enters this quartz crystal or birefringent resin, phases of polarization components parallel to, and perpendicular to the optical axis of quartz crystal can be shifted. Thus, the quartz crystal or birefringent resin can be used as a ¼ or ½ wave plate by setting the shift of phases (phase difference) at a predetermined value.
FIGS. 101 and 102 are perspective diagrams showing the concept of ¼ and ½ wave plates consisting of quartz crystal, respectively. First, referring FIG. 101, with the ¼ wave plate 230 consisting of quartz crystal 231, the thickness of crystal 231 is set whereby the ¼ wave plate 230 has a phase difference of 90° between the polarization components parallel to, and perpendicular to the optical axis of quartz crystal. When linearly polarized light enters this ¼ wave plate 230 consisting of quartz crystal 231 at an inclined angle of about 45° relative to its optical axis, the phase difference between the polarization components perpendicular to each other corresponds about 90°. Thus, the incident linearly polarized light is converted into circularly polarized light as leaving light. Furthermore, referring FIG. 102, with the ½ wave plate 240 consisting of quartz crystal 241, the thickness of crystal 241 is set whereby the ½ wave plate 240 has a phase difference of 180° between the polarization components parallel to, and perpendicular to the optical axis of quartz crystal. Specifically, the quartz crystal 241 has twice the thickness of the quartz crystal 231 composing the ¼ wave plate 230 shown in FIG. 101. When linearly polarized light enters this ½ wave plate 240 consisting of quartz crystal 241 at an inclined angle of about 45° relative to its optical axis, the phase difference between the polarization components perpendicular to each other corresponds about 180°. Thus, the direction of polarization of the incident linearly polarized light is rotated about 90°.
However, when quartz crystal is used as a wave plate as shown in FIGS. 101 and 102, the following disadvantage arises. Namely, a refractive index usually has characteristics that the value varies depending on wavelengths of light (wavelength dispersion characteristics). For this reason, with the ¼ wave plate consisting of quartz crystal designed for light of wavelength about 633 nm, as shown in FIG. 103, if the wavelength of incident light is shifted from the designed wavelength (about 633 nm), there is a disadvantage that the phase difference between the polarization components perpendicular to each other considerably shifts from the phase difference near to 90°. Accordingly, there is a problem that it is difficult to obtain preferable phase conversion characteristics of a wave plate consisting of quartz crystal with respect to light with a wavelength other than designed wavelength.
Furthermore, when a wave plate consisting of quartz crystal is used for an optical device, there is also a problem that it is difficult to improve the characteristics of the optical device. Specifically, with a CD-R/DVD compatible optical pickup device which can be used for both record on CD-R (Compact Disk Recordable), and the reproduction from DVD (Digital Versatile Disk) as a conventional optical device, while a semiconductor laser with wavelength near to 790 nm is used as a semiconductor laser for CD-R, a semiconductor laser with wavelength near to 650 nm is used as a semiconductor laser for DVD. For this reason, when a ¼ wave plate consisting of quartz crystal with the characteristics as shown in FIG. 103 is commonly used for both semiconductor lasers for CD-R and DVD, the following a disadvantage arises. With the common ¼ wave plate, conversion is preferable to the semiconductor laser for DVD with a wavelength near to 650 nm, on the other hand, it is difficult to obtain preferable conversion to the semiconductor laser for CD-R with a wavelength near to about 790 nm. Accordingly, if preferable conversion of the laser light is not obtained by the ¼ wave plate, laser light with the polarization direction other than the designed value is produced. In this case, when the laser light is reflected by a polarization beam splitter, a disadvantage that laser light returns to the semiconductor laser element for CD-R arises. As a result, since optical intensity noise (fluctuation of optical intensity) of the semiconductor laser element increases, a problem that it difficult to improve the characteristics of the CD-R/DVD compatible optical pickup device arises.
Moreover, with a liquid crystal projector device as a conventional optical device, a ½ wave plate is used in order to convert the polarization directions of the white light of a wide wavelength range including red, green and blue components and radiated from a light source into a single polarization direction. In this case, when the ½ wave plate, which consists of quartz crystal with the characteristics similar to the characteristics shown in FIG. 103, is used, the phase difference considerably shifts from the phase difference near to 180° depending on a wavelength, thus, there is a disadvantage that it is difficult to perform preferable conversion for the white light of a wide wavelength range. With the conventional liquid crystal projector device, if conversion of the light is not preferably performed by the ½ wave plate, light with the polarization direction that cannot enters a liquid crystal panel increases, thus, a disadvantage that the efficiency of light utilization reduces arises. As a result, since deviation of color and reduction of luminosity occur caused by reduction of the efficiency of light utilization, there is a problem that it difficult to improve the characteristics of the liquid crystal projector device.
Besides, with the aforementioned conventional CD-R/DVD compatible optical pickup device, a ¼ wave plate, which composed of two transparent substrates, two transparent substrates and a birefringent resin sheet sandwiched between them, is mostly used. In order to obtain the preferable phase conversion characteristic of the ¼ wave plate over a wide wavelength range, the ¼ wave plate with this birefringent resin sheet has two birefringent resin sheets where one sheet is overlaid on another so that they slightly shift in the birefringence direction. However, since a birefringent resin sheet has poor environmental resistance characteristics compared with quartz crystal, it is difficult to use it for a ¼ wave plate of a CD-R/DVD compatible optical pickup device for vehicles. With a liquid crystal projector device, the inside of which becomes high temperature caused by a light source, it is difficult to use a ½ wave plate having a birefringent resin sheet with poor environmental resistance characteristics.